1. Field of the Invention
The invention relates to mass flow meters of the Coriolis-type and, more particularly, to such a mass flow meter for use in sanitary environments.
2. Description of the Prior Art
Coriolis-type mass flow meters are well known in the art and are used to measure the mass and/or density of a fluid or fluid-like material. Representative examples of such mass flow meters are U.S. Pat. Nos. 4,852,410 and 4,756,198. Although details of construction and operation differ somewhat, in general Coriolis-type mass flow meters utilize a pair of tubes disposed between an inlet and an outlet connected in line to a pipe carrying a fluid or fluid-like material (e.g. a slurry) whose mass and/or density is to be measured. External magnetic drivers are used to vibrate the tubes at a characteristic frequency. A change in the frequency measured by motion sensors arranged on the tubes is indicative of the mass flow rate of the fluid travelling through the tubes.
In their usual arrangement, the tubes are held rigidly fixed at one end by an inlet fixture having a Y-shaped passage which causes fluid entering the inlet fixture from an inlet pipe to be split into two substantially equal streams flowing into the fixed ends of the tubes. The opposite ends of the two tubes are also rigidly fixed to an identical outlet fixture having a Y-shaped passage which causes the streams coming from the two tubes to be recombined and applied to an outlet pipe. Such an arrangement is shown in U.S. Pat. No. 4,852,410 and is used in Schlumberger Industries M brand mass flow meter upon which U.S. Pat. No. 4,852,410 is based.
One drawback to the use of such Y-shaped tube fixtures is that the ends of the tubes must be firmly secured within or to the ends of the Y-shaped passages formed in the fixtures. Generally, the tube ends are secured to the Y-shaped passages by means of welding. This poses several problems. First, this requires that the weld between the interior end of the tube and the inside of each Y-shaped passage be made well inside the fixture. This is a particularly difficult operation when smaller sizes of tubing, e.g. one-half inch (13 mm). are used. Second, it is difficult to machine and polish the inside of the Y-shaped passages sufficiently to prevent surface irregularities from interrupting the smooth flow of fluid through the passages and from leaving any areas where contaminants may hide. Finally, the process of creating a weld between the interior of the tube end and the inside wall of each Y-shaped passage often results in a rough edge or lip surrounding the tube end at the well which is also difficult to make smooth and free of places where contamination may reside.
Such drawbacks are especially significant when a mass flow meter of the type described above is to be used in a so-called "sanitary" environment. For example, mass flow meters may be used to measure the mass and/or density of milk products, food-stuffs, or the like. Metering equipment of this type must meet various sanitary standards and regulations such as the "3-A" Sanitary Standard No. 28-01 of the Sanitary Standards Symbol Administration Council. These standards and regulations require that the metering mechanism be free of areas or obstructions where foodstuffs or contaminants can hide, and that the metering mechanism be easily removable from the fluid flow pipeline to allow for easy cleaning, sanitization, and inspection.